CVE-2024-39894

Executive Summary

CVE-2024-39894 is a timing side-channel vulnerability in OpenSSH versions 9.5 through 9.7. The flaw resides in the ObscureKeystrokeTiming feature, which was designed to obfuscate keystroke timing patterns. Due to a logic error, the feature fails to properly obscure timing when echo is disabled (e.g., during password entry for su and sudo). This allows a remote attacker to perform a timing attack to infer keystroke timing, potentially recovering passwords or other sensitive input. The vulnerability has a CVSS score of 0.0 (pending official scoring) but is assessed as low severity due to the high complexity of exploitation and limited information disclosure. No public proof-of-concept or exploit code is available at this time.

Supply Chain & Package Security

This vulnerability affects the OpenSSH package, which is a critical component in virtually all Linux and Unix-like operating systems. It is distributed through official package repositories (e.g., apt, yum) and as source code. Users should verify they are not running vulnerable versions (9.5, 9.6, 9.7) and apply the patch by upgrading to 9.8 or later. No malicious packages have been reported in relation to this CVE.

CVSS Breakdown & Threat Level

CVSS Score: 0.0 (Pending official scoring from NVD) Assessed Severity: Low

Attack Vector (AV): Network (N) – The attacker can be remote. Attack Complexity (AC): High (H) – Requires precise timing measurements and multiple attempts. Privileges Required (PR): None (N) – No authentication needed to trigger the vulnerability. User Interaction (UI): None (N) – The victim does not need to perform any action beyond normal password entry. Scope (S): Unchanged (U) – The vulnerability does not affect other components. Confidentiality (C): Low (L) – Limited information disclosure (keystroke timing). Integrity (I): None (N) – No data modification. Availability (A): None (N) – No denial of service.

Threat Level: Low. Exploitation requires a high level of skill and network proximity. The information leak is limited to timing patterns, which may not directly reveal passwords without additional analysis.

Deep Dive & Mechanics

The ObscureKeystrokeTiming feature in OpenSSH was introduced to prevent keystroke timing analysis by adding random delays to packet transmission. The feature works by buffering keystrokes and sending them in packets with randomized timing, making it harder for an attacker to infer what is being typed based on inter-keystroke intervals.

The Logic Error: When echo is disabled (e.g., during password entry), the feature incorrectly handles the timing obfuscation. Specifically, the code that decides when to send a packet does not account for the echo-off state, causing the timing of packets to correlate with the actual keystroke timing. This allows an attacker who can measure network round-trip times or packet arrival times to deduce the intervals between keystrokes.

Attack Scenario:

1. An attacker establishes a man-in-the-middle (MITM) position or is on the same network segment as the victim.
2. The victim connects via SSH to a server and runs su or sudo, which prompts for a password with echo disabled.
3. The attacker captures the timing of SSH packets during password entry.
4. By analyzing the inter-packet delays, the attacker can infer the timing of keystrokes, which can be used to reduce the search space for password guessing (e.g., distinguishing between short and long pauses, or detecting special characters).

Affected Versions:

• OpenSSH 9.5
• OpenSSH 9.6
• OpenSSH 9.7

Fixed Version:

• OpenSSH 9.8

Vulnerable vs. Patched Code

The vulnerable code is in the ssh client's handling of keystroke timing. The patch modifies the logic in the obscure_keystroke_timing function to ensure that when echo is off, the timing obfuscation is applied correctly.

Vulnerable Code (simplified):

// In sshconnect.c or similar
void obscure_keystroke_timing(int echo_on) {
    if (echo_on) {
        // Apply obfuscation
        send_packet_with_delay();
    } else {
        // Logic error: no obfuscation applied
        send_packet_immediately();
    }
}

Patched Code (simplified):

// In sshconnect.c or similar
void obscure_keystroke_timing(int echo_on) {
    // Always apply obfuscation, regardless of echo state
    send_packet_with_delay();
}

The actual patch is more nuanced, involving changes to the packet scheduling logic to ensure consistent timing behavior.

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• Network access to the victim's SSH traffic (MITM or same subnet).
• Ability to capture and analyze packet timestamps with microsecond precision.
• Victim using OpenSSH 9.5-9.7 and entering a password via su or sudo.

Lab Setup:

1. Set up two machines: Attacker (Kali Linux) and Victim (Ubuntu 22.04 with OpenSSH 9.5).
2. On the victim, install the vulnerable OpenSSH version.
3. On the attacker, use tcpdump or Wireshark to capture SSH traffic.

Exploitation Steps:

1. Initiate an SSH session from the victim to a server (or localhost).
2. On the victim, run sudo -i or su - and enter a password.
3. On the attacker, capture the SSH packets during password entry.
4. Analyze the inter-packet timing using a script (e.g., Python with scapy).

Example Analysis Script:

from scapy.all import *
import numpy as np

packets = rdpcap('ssh_capture.pcap')
timestamps = [p.time for p in packets if p.haslayer(TCP) and p[TCP].dport == 22]
intervals = np.diff(timestamps)
print("Inter-keystroke intervals (seconds):", intervals)

Note: No public exploit code exists. This is a theoretical attack.

Blue Team (Defensive)

Detection:

• Monitor for unusual SSH traffic patterns, especially during su/sudo usage.
• Use network monitoring tools to detect timing analysis attempts (e.g., repeated SSH connections with precise timing measurements).

Mitigation:

• Upgrade to OpenSSH 9.8 or later.
• If upgrade is not possible, disable the ObscureKeystrokeTiming feature (if configurable) or use alternative authentication methods (e.g., SSH keys).
• Implement network segmentation to reduce MITM risk.

Indicators of Compromise (IOCs)

• Network: Unusual SSH traffic with consistent inter-packet timing patterns during password entry.
• Host: OpenSSH version 9.5, 9.6, or 9.7 installed.
• Logs: No specific log entries for this vulnerability; detection requires network analysis.

File Hashes (for vulnerable binaries):

• OpenSSH 9.5: sha256:abc123... (example)
• OpenSSH 9.6: sha256:def456... (example)
• OpenSSH 9.7: sha256:ghi789... (example)

Note: Actual hashes vary by distribution and build.

Detection Engineering

YARA Rule

rule CVE_2024_39894_OpenSSH_Timing_Attack {
    meta:
        description = "Detects vulnerable OpenSSH versions 9.5-9.7"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2024-39894"
    strings:
        $v9_5 = "OpenSSH_9.5" nocase
        $v9_6 = "OpenSSH_9.6" nocase
        $v9_7 = "OpenSSH_9.7" nocase
    condition:
        any of them
}

Sigma Rule

title: CVE-2024-39894 - OpenSSH Timing Attack Detection
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects SSH connections using vulnerable OpenSSH versions 9.5-9.7
author: Valters IT
date: 2025-04-10
logsource:
    category: network_connection
    product: zeek
detection:
    selection:
        service: ssh
        software|contains:
            - 'OpenSSH_9.5'
            - 'OpenSSH_9.6'
            - 'OpenSSH_9.7'
    condition: selection
falsepositives:
    - Legitimate use of older OpenSSH versions
level: low

Suricata Rule

alert tcp any any -> any 22 (msg:"CVE-2024-39894 - OpenSSH Timing Attack - Vulnerable Version Detected"; flow:to_server,established; content:"OpenSSH_9.5"; nocase; sid:1000001; rev:1;)
alert tcp any any -> any 22 (msg:"CVE-2024-39894 - OpenSSH Timing Attack - Vulnerable Version Detected"; flow:to_server,established; content:"OpenSSH_9.6"; nocase; sid:1000002; rev:1;)
alert tcp any any -> any 22 (msg:"CVE-2024-39894 - OpenSSH Timing Attack - Vulnerable Version Detected"; flow:to_server,established; content:"OpenSSH_9.7"; nocase; sid:1000003; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade OpenSSH to version 9.8 or later on all systems.
2. If upgrade is not possible, consider disabling the ObscureKeystrokeTiming feature by setting ObscureKeystrokeTiming no in ssh_config (note: this may reduce security against other timing attacks).
3. Use SSH key-based authentication instead of passwords for su/sudo (e.g., via sudo with NOPASSWD or SSH_AUTH_SOCK).

Long-Term Measures:

• Implement network segmentation to prevent MITM attacks.
• Monitor for SSH version banners and alert on vulnerable versions.
• Conduct regular vulnerability scanning.

Patch Details:

• Official patch: Upgrade to OpenSSH 9.8 (released 2024-07-01).
• Distribution-specific patches: Check your OS vendor (e.g., Ubuntu, Red Hat) for backported fixes.

Valters IT